On the Polar Stereographic grids, the vector wind is resolved
into u and v components with respect to the grid coordinates, i.e., u
represents motion in the direction of increasing x (i) coordinate, v in
the direction of increasing y (j). On the latitude-longitude grids, u
and v are true eastward and northward components, respectively.
However, take note of Table 7, below, which allows for the
specification of other possibilities when the Grid Description Section
is included in the message.

(i) The grid points are laid out in a linear array such that
the longitude index (the columns) is the most rapidly varying. For the
northern hemisphere grids the first point in the record is at the
intersection of the western-most meridian and southern-most circle of
latitude; the last point is the single Polar value (see note iii,
below). For the southern hemisphere grids the first point in the record
is the single Polar value (see note iii, below); the last point is at
the intersection of the eastern-most meridian and northern-most circle
of latitude. For those familiar with FORTRAN subscripting conventions,
longitude is the first subscript, latitude the second.

(ii) In grids 21 through 26, and 61 through 64, the values on
the shared boundaries are included in each area.

(iii) The datum for the pole point is given only once in each
grid. The user must expand, if desired, the single pole point value to
all the pole "points" at the pole row of a latitude-longitude grid.
Scalar quantity values are the same for all pole points on a the grid.
Wind components at the poles are given by the formulae:

u = -speed * sin(dd) & v = -speed * cos(dd)

where dd is the direction of the wind as reported according to the
specification of wind direction at the poles (refer to WMO Manual on
Codes, code table 878).

The WMO convention can be given this operational definition:
At the North Pole, face into the wind and report the value of the west
longitude meridian along which the wind is coming at you; at the South
Pole do likewise but report the east longitude meridian value. This is
equivalent to placing the origin of a right-handed Cartesian coordinate
system on the North Pole with the y-axis pointing to the prime (0
degree) meridian and the x-axis pointing to the 90 degrees west
meridian, and then resolving any vector wind at the pole point into
components along those axes. At the South Pole the coordinate axes are
oriented such that the y-axis points toward 180 degrees west. Those
components are the u- and v-values given as the single pair of pole
point winds in the GRIB format.

In terms of a longitude/latitude grid these are the wind
components for the pole point at the 180 degree meridian. For example,
on a 2.5x2.5 degree northern hemisphere grid (145x37 points), with the
abscissa along the equator and the ordinate along the prime meridian,
the transmitted north pole wind components are those that belong at the
gridpoint (73,37). The wind components at the other grid points along
the pole row may be obtained through suitable rotation of the
coordinate system. All the components at the pole row are, of course,
simply representations of the same vector wind viewed from differing
(rotated) coordinate systems. In the southern hemisphere the analogous
situation holds; the single set of transmitted pole point wind
components belong at the gridpoint (73,1).

(iv) Grid 50 is a set of points over the
contiguous United States and environs on a grid extending from 20N (row
No. 1) to 60N (row No. 33) in 1.25 degree intervals. The grid increases
in longitudinal extent from south to north in the following manner:

In the figure the coordinates indicate the location of the
octants of the globe, the numbers are the corresponding grid
identification numbers (PDS Octet 7), and the letters are the grid
identification used in the WMO heading (see Appendix A).

The left and right meridional columns of each octant/grid are
shared with the neighbors.

The basic grid point separation is 1.25x1.25 deg. on a
latitude/longitude array, but the grid is "thinned" by reducing the
number of points in each row as one goes northward (or southward) away
from the equator. In GRIB terms, this is referred to as a
"quasi-regular" grid.

The latitudinal increment is always 1.25 deg.; this results
in 73 rows where the pole is included as a "row", not a single
gridpoint.

The longitudinal spacing at the equator is also 1.25 deg.;
thus there will be 73 gridpoints at the equator in each octant.

The number of points on each latitudinal row, other than the
equator, is given by (using FORTRAN notation):

NPOINTS = IFIX(2.0 + (90.0/1.25) * COS(LATITUDE))

Thus at the pole there will be two gridpoints, one each at
the meridians that delineate the edges of the octant. The formula was
worked out so that there is (approximately) equal geographic separation
between the grid points uniformly across the globe.

Because of variations in precision and roundoff error in
different computers, the value of NPOINTS may vary by 1 at "critical"
latitudes when calculated on various hardware platforms. Here is a
table of the exact values of NPOINTS as a function of latitude as used
in the internationally exchanged grids. These numbers will, of course,
be found in the Grid Description Section of each GRIB bulletin.

Latitude Range
inclusive
(north or south)

NPOINTS

Latitude Range
inclusive
(north or south)

NPOINTS

0.00 - 8.75

73

55.00

43

10.00 - 12.50

72

56.25

42

13.75 - 16.25

71

57.50

40

17.50 - 18.75

70

58.75

39

20.00 - 21.25

69

60.00

38

22.50

68

61.25

36

23.75 - 25.00

67

62.50

35

26.25

66

63.75

33

27.50 - 28.75

65

65.00

32

30.00

64

66.25

30

31.25

63

67.50

29

32.50

62

68.75

28

33.75

61

70.00

26

35.00 - 36.25

60

71.25

25

37.50

59

72.50

23

38.75

58

73.75

22

40.00

57

75.00

20

41.25

56

76.25

19

42.50

55

77.50

17

43.75

54

78.75

16

45.00

52

80.00

14

46.25

51

81.25

12

47.50

50

82.50

11

48.75

49

83.75

9

50.00

48

85.00

8

51.25

47

86.25

6

52.50

45

87.50

5

53.75

44

88.75

3

90.00

2

When all this is put together the result is that there are
3447 points of data actually transmitted in any individual GRIB
bulletin containing one octant of the globe.

In the GRIB bulletins all of this information will be
included in the Grid Description Section (GDS); the GDS must be
included in order to describe the thinned or "quasi-regular" grid
structure. See Section 2 and Table C for the general description of the
GDS; what follows are the specific values of the variables in the GDS
that describe these eight grids.

GDAS Contents

Octets

Value or variable

1-3

178 (length of GDS)

4

0 (or 255, either indicating no PV)

5

33 (pointer to start of PL list)

6

0

7-32

Grid description - see below

33-178

number of points in each of 73 rows (2 octets per point)

Details of Octets 7-32 - Grid Description

Octets

Variable & Value

7-8

Ni = all bits set to 1 (missing)

9-10

Nj = 73

GRID:

37

38

39

40

41

42

43

44

La1 =

0

0

0

0

90S

90S

90S

90S

Lo1 =

330

60

150

240

330

60

150

240

11-13

14-16

17

Resolution & Component Flag = [10000000] (binary)

GRID:

37

38

39

40

41

42

43

44

La2 =

90N

90N

90N

90N

0

0

0

0

Lo2 =

60

150

240

330

60

150

240

330

18-20

21-23

24-25

Di = all bits set to 1 (missing)

26-27

Dj = 1.25 deg

28

Scan Mode = [01000000] (binary)

29-32

Set to 0 (unused)

Note that the scanning direction is from the bottom (south
edge) to the top of the octant grids, regardless of the hemisphere.
Thus in the northern hemisphere the first 73 data points (in the BDS)
will be the equatorial values and the last two will be the Polar
values. The PL counts in the GDS octets 33-178 will, of course,
indicate contain these numbers.

In the southern hemisphere, the first two data points will be
the south pole values, and the last 73 points will be the equatorial
values. Octets 33-34 in the GDS will contain "2", octets 35-36 will
contain a "3", and so on to octets 177-178 which will contain "73".

Note: Grids 160-175 below are intended for use in the U.S.
Weather Service's Advanced Weather Information Processing System
(AWIPS). Their definition is subject to change as the AWIPS
requirements are further refined. The parenthetical letters adjacent to
the numeric values are the WMO header identification of the grid for
headers starting with "Y" or "Z". For headers starting with "O", the
bracketed letter is the WMO header identification for oceanographic
grids. See
appendix A.

Note: The following grids are intended for use in the U.S.
Weather Service's Advanced Weather Information Processing System
(AWIPS). Their definition is subject to change as the AWIPS
requirements are further refined. The parenthetical letters adjacent to
the numeric values are the WMO header identification of the grid for
headers starting with "Y" or "Z". For headers starting with "O", the
bracketed letter is the WMO header identification for oceanographic
grids. See
appendix A.

The longitudinal grid spacing is 0.766 degrees. The grid is positioned
such that the odd-numbered rows and columns coincide with the National
grid, No. 204; the lower left corner of the regional grid is located at
National (204) grid-point (55,24) and the upper right corner is located
at (69,37).